Industry continues to demand miniature pressure transducers which are capable of operating at high temperatures in excess of 800.degree. C. Such transducers are extremely desirable for use in satellites, nuclear power, chemical processing, aerodynamics, engine testing, and other applications.
As is well known in the art, pressure transducers generally include a force collector and one or more piezoresistive sensor elements. Many different types of pressure transducer structures have been proposed to increase the reliability and accuracy of such devices in high temperature applications.
For example, some pressure transducers employ a metal force collector which includes one or more sensor elements bonded thereto with an epoxy. The sensitivity of such a transducer is initially relatively low and degrades further when the transducer is subjected to high temperature operating environments. Other pressure transducer designs employ silicon (Si) force collectors with integrally formed pn junction isolated Si sensor elements. Such pressure transducers can operate up to approximately 350.degree. C. before the pn junction isolation deteriorates. Still other pressure transducer designs utilize Si sensor elements which are dielectrically isolated from a Si force collector. Examples of such transducer designs can be seen in U.S. Pat. Nos. 3,800,264 and 3,930,823, both entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS, and both issued to A.D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. The pressure transducers described therein are capable of operating in excess of 500.degree. C. When the operating temperatures exceed 600.degree. C., however, the Si sensor elements and the Si force collector undergo plastic deformation which renders the transducer inoperable.
Other materials such as silicon carbide (SIC) have been employed in the fabrication of pressure transducers to increase their operating temperature. The use of SiC in pressure transducers is described in detail in U.S. Pat. No. 5,165,283 entitled HIGH TEMPERATURE TRANSDUCERS AND METHODS OF FABRICATING THE SAME EMPLOYING SILICON CARBIDE, issued to A.D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein. This patent describes a pressure transducer which employs pn junction isolated SiC sensing elements on a SiC force collector. Such a pressure transducer is capable of operating at temperatures greater than 600.degree. C. However, the SiC material from which the sensing elements are formed, exhibits a relatively low gauge factor which is approximately 31. The gauge factor of the sensing elements control the output signal of the pressure transducer, which is an important operating parameter of such a device. Generally, sensing elements fabricated from a material exhibiting a higher gauge factor yields a higher output signal level than sensing elements fabricated from a material exhibiting a lower gauge factor.
In order to increase the output signal of a SiC pressure transducer, the prior art has proposed fabricating the sensing elements from a polycrystalline diamond (poly-diamond) film. A pressure transducer having a SiC force collector and sensing elements fabricated from a piezoresistive (PZR) polycrystalline diamond film is described in U.S. Pat. No. 5,303,594 entitled PRESSURE TRANSDUCER UTILIZING DIAMOND PIEZORESISTIVE SENSORS AND SILICON CARBIDE FORCE COLLECTOR, issued to A.D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc., the assignee herein.
Polycrystalline diamond (poly-diamond) films are known to possess excellent thermal properties. Moreover, poly-diamond has a gauge factor of approximately 100, which is substantially higher than the gauge factor of SiC which is 31, as stated earlier above.
The ability to grow diamond films has also open the door to the fabrication of diamond-based semiconductor diodes and transistors as described in an article entitled "DIAMOND FILM SEMICONDUCTORS" by Michael W. Geis et al., SCIENTIFIC AMERICAN, October, 1992. Another application for diamond films has been in the area of injection lasers. In laser applications, diamond films have been employed to replace copper as a heat sink for mounting a laser, since diamond has five times the thermal conductivity of copper. Other applications for diamond films include power devices and applications where film materials are used for providing high abrasion resistance and high impact strength.
The diamond films employed in the applications discussed above, have some disadvantages which up to now, have not been successfully addressed in the prior art. One disadvantage is that diamond films start oxidizing at approximately 600.degree. C. and disintegrate quickly at temperatures approaching 800.degree. C. Another disadvantage associated with diamond films is that diamond films are extremely difficult to pattern into semiconductor and piezoresistive circuit patterns, thus, complicating the fabrication of these structures. Up until now, conventional reactive ion etching (RIE) and lift-off patterning techniques have been used. Patterning poly-diamond films using RIE is very costly, time consuming and cumbersome. Similarly, the lift-off is difficult and places limits on the quality of the diamond films due to the survivability limits of the lift-off films during the diamond deposition process.
It is, therefore, an object of the present invention to provide a method for substantially preventing the oxidation and disintegration of diamond films at temperatures above 600.degree. C.
It is another object of the present invention to provide an improved method for patterning diamond films which substantially overcomes the problems of prior art techniques.